1. Field of the Invention
This invention relates to a power amplifier of push-pull operation for use in a portable telephone, etc.
2. Related art of the Invention
The configuration of a push-pull power amplifier hitherto implemented on the same semiconductor chip may be shown in a block diagram of FIG. 5. Referring to FIG. 5, an input signal from an input terminal 51 is split through an input side balun circuit 52 into two signals each having equal power and phase difference of 180 degrees with respect to each other. For one of the split signals, a first input matching circuit 53 matches impedance and the signal is amplified by a first transistor 54. Then, a first output matching circuit 55 matches impedance for the signal and then the signal is inputted to one of the two inputs of an output side balun circuit 56. In the same way, a second input matching circuit 57 matches impedance for the other of the split signals and the signal is amplified by a second transistor 58. Then, a second output matching circuit 59 matches impedance for the signal, and the signal is inputted to the other input of the output side balun circuit 56.
The output side balun circuit 56 makes phase difference of 180 degrees between the input from the first output matching circuit 55 and that from the second output matching circuit 59, so that the two inputs are combined in phase and a combined signal is outputted from an output terminal 60.
However, a push-pull power amplifier with the configuration shown in FIG. 5 has such a problem that the efficiency reduces at a low output level. That is, an amplifier in general has a characteristic of high efficiency at the maximum output level, and so there is a problem that both amplifiers 54 and 58 are of low efficiency at a lower output level.